1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, laser printer or similar image forming apparatus and, more particularly, to a bistage type developing device having a photoconductive drum or similar image carrier, a developing roller or similar first toner conveying means, and a roller or second conveying means intervening between the image carrier and the first toner conveying means.
2. Discussion of the Background
It is a common practice with a developing device to use toner of high electric resistance, as distinguished from a toner and carrier mixture. This kind of developing devices are generally classified into three types, i.e., an S-NSP type device using a soft developing roller, a .mu.-ISP type device using a hard developing roller, and a bistage .mu.-ISP type device having second toner conveying means between a photoconductive element or similar image carrier and first toner conveying means.
In the S-NSP type device, toner deposited on the developing roller is regulated by a blade to form a thin layer and then conveyed to a photoconductive drum or image carrier. The drum is made of a hard material. Hence, the developing roller is made of rubber or similar soft material. As a result, a nip necessary for development is formed between the roller and the drum. On the other hand, in the .mu.-ISP type device, the toner is leveled by the blade and charged mainly by the friction between the blade and the toner and the friction between the particles of the toner themselves. Because the developing roller is hard, use is made of a photoconductive belt. The hard roller and belt form a nip therebetween. Further, in the bistage .mu.-ISP type device, second toner conveying means in the form of a belt is interposed between the first toner conveying means and the image carrier, as taught in Japanese Patent Laid-Open Publication No. 61-34557 by way of example.
However, the S-NSP type device using a soft developing roller has various problems, as follows.
(1) The soft developing roller makes it difficult for the blade to form a uniform thin toner layer thereon.
(2) The soft developing roller is apt to suffer from creep deformation (perpetual compression distortion) and fail to uniformly contact the blade and image carrier, resulting in defective images.
(3) It is difficult to uniformly charge the toner. As a result, toner particles charged to the opposite polarity appear and contaminate the background of images.
The above problems (1) and (2) are attributable to the soft developing roller and, therefore, do not occur in the .mu.-ISP type device. However, the .mu.-ISP type device brings about the following drawbacks.
(4) The photoconductive belt must be accompanied by a belt drive mechanism, including a drive roller and gears, which increases the cost.
(5) The photoconductive belt becomes offset to either side due to, for example, the irregular tension distribution of the belt. Hence, a mechanism for preventing such a n occurrence is required.
The toner particles charged to the opposite polarity, as in the above problem (3), occur even when use is made of a hard developing roller. The bistage .mu.-ISP type device is a measure proposed against the oppositely charged toner particles. This type of device, however, also has the problems stated in relation to the .mu.-ISP type device using a hard developing roller because the second toner conveying means is implemented as a belt.
In light of the above, a developing device having a second toner conveying means in the form of a roller has already been proposed. Although this kind of device obviates the problem attributable to the oppositely charged toner at low cost, it is not fully satisfactory in respect of the prevention of defective images.